ATAR Notes: Forum
VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: Andiio on April 13, 2011, 07:29:52 pm
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Just as a confirmation to whether I'm correct, how does a lower/higher solubility affect the retention time of components?
Thanks!
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If it is soluble in water it is polar and thus if you have a polar stationary phase it will adsorb more strongly to it and increase retention time. If the mobile phase is a polar solvent then obviously the retention time will decrease.
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In HPLC, the higher the solubility of the component to the mobile phase (liquid) the shorter the retention time is for that component. In GLC, the higher the solubility of the component to the stationary phase (liquid) the longer the retention time is for that component.
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What if you have a non-polar liquid Zien?
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It'll be the other way round. If the component was non-polar, then you'd use a non-polar solvent so dispersion forces can act to dissolve the component. You wouldn't have a polar component and then use a non-polar solvent. :p
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Sounds good to me!
Basically, the important thing to remember with this is that non-polar adsorbs strongly to non-polar and polar adsorbs strongly to polar. After that, size of molecule and functional groups can alter retention time.
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Just to confirm my own suspicions, you have to follow the like dissolves like rule for the component and mobile phase, but what about the stationary phase?
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In HPLC, the higher the solubility of the component to the mobile phase (liquid) the shorter the retention time is for that component. In GLC, the higher the solubility of the component to the stationary phase (liquid) the longer the retention time is for that component.
Do you think you could explain either of those?
I thought (in GLC) a lower solubility in the stationary phase would yield a lower retention time, and yeah I agree with you on the HPLC one.
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Chromatography is like a tug-of-war between the stationary and liquid phases. No matter how great the solubility/adsorption the component has to any phase, it will eventually be swept by the movement of the mobile phase if nothing else.
Let's say we have two components in a gaseous mixture to be distinguished using GLC: one component is polar (A) and the other component is non-polar (B). Let's also say that the stationary phase (liquid) is also polar. Both components would be swept away by the non-reactive gas (e.g. nitrogen). Since A is polar and the stationary phase is polar, it would dissolve in the stationary liquid phase for some time before being swept away by the nitrogen gas. Then it dissolves in the stationary liquid phase again and continues this cycle until it reaches the detector. B is a non-polar gas, unlikely to be influenced by the stationary liquid. It'll just zip along down the column until it reaches the detector.
The important thing to keep note here is that it repeats this cycle of adsorption and desorption. The more soluble the component is to the stationary phase, the longer the retention time (as it spends longer amounts of time dissolve and not moving).
Just to confirm my own suspicions, you have to follow the like dissolves like rule for the component and mobile phase, but what about the stationary phase?
The stationary phase could be polar as well as the mobile phase being polar in a HPLC. That's when things really start to go wild and we have to go "As a general rule of thumb..". :p
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Just to confirm my own suspicions, you have to follow the like dissolves like rule for the component and mobile phase, but what about the stationary phase?
The stationary phase could be polar as well as the mobile phase being polar in a HPLC. That's when things really start to go wild and we have to go "As a general rule of thumb..". :p
I wonder who you're quoting there Zien... Lol.
Just to clarify what Zien's saying there, what he means is yes, you do have to follow the "like dissolves like" rule if you have a polar component and a polar stationary liquid phase (in HPLC), and that if all three things (ie the mobile phase, stationary phase and component) are the same, then as your "general rule of thumb" the polarity of the mobile phase is more influential than that of the stationary phase - ie, if everything is polar or non-polar, then the mobile phase "wins" the tug of war.
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Wrong way round, Aurelian. XD
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Ok, thanks both of you. Just on a final note, and just to check we are all on the same page, would you care to fill in the following (I made it up myself :P):
Assume we have a (P) Polar Component and a (N) Non Polar Component. What types of phases (Polar/Non-Polar) would be best to use for HPLC and GLC if we wanted:
a) The P component to have the shortest retention time
a) The N component to have the shortest retention time
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(I'm assuming the second part was supposed to be b) XD)
a) HPLC - Polar Mobile (liquid) phase and non-polar stationary phase
GLC - Non-polar stationary phase (liquid)
b) HPLC - Non-polar mobile (liquid) phase and polar stationary phase
GLC - Polar stationary phase (liquid)
Actually, the issue of choosing solids for HPLC and GLC was something that we didn't go too deep on. Don't take my word on it for sure unless someone who really knows their stuff says that this is generally how they do it. :p I'm not absolutely confident~
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Yay that's what I got :P
You'd do the same as well for replacing P with Very Polar, and N with Slightly Polar, or is it not the case?
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Wrong way round, Aurelian. XD
Hmmm was it? I swear she said mobile phase won... actually maybe she said stationary phase won... =|
This is why I don't particularly like "rules of thumb" because you can't derive them deductively from anything... >_>
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This is why I don't particularly like "rules of thumb" because you can't derive them deductively from anything... >_>
Agreed. I don't think there can be a 'rule of thumb' for this. Polarity is a sketchy enough concept as is. This is my reply to a question received via pm. Reposted here for anyone interested:
Paper is made of cellulose, and cellulose is polar.
Does this mean that the mobile phase will also adsorb to the cellulose is what i am wondering.
Just another follow up question,
the size of the substance would not be as relevant Cf the intramolecular forces at play are, would it?
For example, a larger component of a substance, that is polar will have a higher than a different small component of the substance which is non-polar, correct? (assuming our solvent/M.P is also polar)
But in the case where the same large component is tested twice, once where it is of low concentration and one where it is of high concentration, will the lower concentrated sample move slower?
I thought that since it was large, it would have more polar areas on it to desorb to the water, and thus a larger concentrated component would move faster.
You are correct in saying that cellulose is polar, thus both mobile phase and stationary phase are polar. It is logical to say interaction between solvent and stationary phase will be strong.
However, you should realise that separation is not as simple as dipolar interaction. Any type of attractive interaction will play a part. This includes dispersion forces and so on. The rate of adsorption and desorption really depend on how the interaction plays out. Solubility can play a factor. You already know that solubilities are rather 'unpredictable'. So when both mobile phase and stationary phase are polar, it is quite hard to predict what comes out first. The ability to H-bond is also important for both phases.
Generally, larger molecules prefer to stick with the stationary phase (stronger dispersion interaction). Larger molecules also tend to be less soluble than smaller molecules. I would try to give a more complete picture, but the actual interaction model behind it is quite complicated.
Concentration generally does not affect retention. Size does affect retention.
So what I'm really saying is there isn't a reliable way to tell. You have to consider the polarity, as well as whether the solvent is protic or not, and whether your eluent is protic or not. At this point, it is far easier to do the experiment, characterise the results and come up with a theory than apply the theory to predict the results.